How close to detailed spectral calculations is the k-distribution method and correlated-k approximation of Kato et al. (1999) in each spectral interval?

International audienceThe k-distribution method and the correlated-k approximation of Kato et al. (1999) is a smart approach originally designed for broadband calculations of the solar radiation at ground level by dividing the solar spectrum in 32 spectral bands. The approach is a priori not suited for calculation of spectral irradiance. Nevertheless, this paper evaluates its performance when compared to more detailed spectral calculations serving as references for the spectral intervals no. 3 [283, 307] nm to 26 [1613, 1965] nm for clear and cloudy situations. The evaluation is based on numerical simulations. The clearer the sky, the greater the root mean square error (RMSE) in all bands. In the spectral intervals no. 3 and 4 [307, 328] nm, the irradiance is underestimated by large – approximately −90 % and −17 % in relative value -because the wavelength interval is large with respect to the absorption by ozone and a single value of ozone cross section is not enough for each interval. For each spectral interval from no. 5 [328, 363] nm to no. 18 [743, 791] nm, and for both global and direct radiation, the bias and the RMSE are less than 1.5 % of the irradiance in the corresponding interval under clear skies and may amount to 3 % in cloudy conditions. For greater wavelength intervals no. 19 to no. 26, the relative bias and RMSE show a tendency to increase with wavelength and may reach 8 % and 7 % for global and direct under clear skies respectively, and 11 % and 15 % under cloudy skies

Estimating the photosynthetically active radiation under clear skies by means of a new approach

International audienceThe k-distribution method and the correlated-k approximation of Kato et al. (1999) is a computa-tionally efficient approach originally designed for calculations of the broadband solar radiation by dividing the solar spectrum in 32 specific spectral bands from 240 to 4606 nm. This paper describes a technique for an ac-curate assessment of the photosynthetically active radiation (PAR) from 400 to 700 nm at ground level, under clear-sky conditions using twelve of these spectral bands. It is validated against detailed spectral calculations of the PAR made by the radiative transfer model libRadtran. For the direct and global PAR irradiance, the bias is −0.4 W m−2 (−0.2 %) and −4 W m−2 (−1.3 %) and the root mean square error is 1.8 W m−2 (0.7 %) and 4.5 W m−2 (1.5 %). For the direct and global Photosynthetic Photon Flux Density, the biases are of about +10.3 µmol m−2 s−1 (+0.8 %) and 1.9 µmol m−2 s−1 (−0.1 %) respectively, and the root mean square error is 11.4 µmol m−2 s−1 (0.9 %) and 4.0 µmol m−2 s−1 (0.3 %). The correlation coefficient is greater than 0.99. This technique provides much better results than two state-of-the-art empirical methods computing the daily mean of PAR from the daily mean of broadband irradiance

A new approach for estimating operationally the spectral distribution of surface solar irradiance: preliminary results

International audienceThe k-distribution method and the correlated-k approximation of Kato et al. (1999) are a smart approach designed for calculations of the broadband solar radiation at ground level by using only 32 spectral bands. This communication presents a preliminary assessment of the performance of this approach compared to more detailed spectral calculations for the spectral intervals no. 3 [283, 307] nm to no. 26 [1613, 1965] nm for clear and cloudy situations. For spectral intervals no. 5 [328, 363] nm to no. 26, the relative errors are less than 5% of the irradiance in the corresponding interval. The irradiance is strongly underestimated in the spectral intervals no. 3 and no. 4 [307, 328] nm. It is found that if necessary, errors may be accurately corrected with simple models computed only once

Smart Approaches for Evaluating Photosynthetically Active Radiation at Various Stations Based on MSG Prime Satellite Imagery

Photosynthetically active radiation (PAR) is the 400–700 nm portion of the solar radiation spectrum that photoautotrophic organisms including plants, algae, and cyanobacteria use for photosynthesis. PAR is a key variable in global ecosystem and Earth system modeling, playing a prominent role in carbon and water cycling. Alongside air temperature, water availability, and atmospheric CO2 concentration, PAR controls photosynthesis and consequently biomass productivity in general. The management of agricultural and horticultural crops, forests, grasslands, and even grasses at sports venues is a non-exhaustive list of applications for which an accurate knowledge of the PAR resource is desirable. Modern agrivoltaic systems also require a good knowledge of PAR in conjunction with the variables needed to monitor the co-located photovoltaic system. In situ quality-controlled PAR sensors provide high-quality information for specific locations. However, due to associated installation and maintenance costs, such high-quality data are relatively scarce and generally extend over a restricted and sometimes non-continuous period. Numerous studies have already demonstrated the potential offered by surface radiation estimates based on satellite information as reliable alternatives to in situ measurements. The accuracy of these estimations is site-dependent and is related, for example, to the local climate, landscape, and viewing angle of the satellite. To assess the accuracy of PAR satellite models, we inter-compared 11 methods for estimating 30 min surface PAR based on satellite-derived estimations at 33 ground-based station locations over several climate regions in Europe, Africa, and South America. Averaged across stations, the results showed average relative biases (relative to the measurement mean) across methods of 1 to 20%, an average relative standard deviation of 25 to 30%, an average relative root mean square error of 25% to 35% and a correlation coefficient always above 0.95 for all methods. Improved performance was seen for all methods at relatively cloud-free sites, and quality degraded towards the edge of the Meteosat Second Generation viewing area. A good compromise between computational time, memory allocation, and performance was achieved for most locations using the Jacovides coefficient applied to the global horizontal irradiance from HelioClim-3 or the CAMS Radiation Service. In conclusion, satellite estimations can provide a reliable alternative estimation of ground-based PAR for most applications

Towards an automatic method for estimating the spectral distribution of the solar radiation in clear sky conditions. Application cases : UV, photosynthesis, daylight

Nous nous intéressons à l'estimation de la distribution spectrale de l'éclairement solaire au sol en tout lieu et tout instant. Etant donné le très petit nombre de stations de mesure au sol, nous nous proposons de développer une méthode numérique exploitant des estimations de la composition atmosphérique faites quotidiennement. Cette méthode doit être suffisamment rapide pour être utilisée en mode intensif de manière opérationnelle. Les domaines d'applications ciblés sont l'ultraviolet (UV), la photosynthèse et la lumière du jour, avec une limitation au cas du ciel clair. Notre première innovation est de partir paradoxalement des approches numériques très rapides et précises de calcul de l'éclairement total. Ces approches développées donnent une description de la distribution spectrale du rayonnement solaire en un nombre restreint de bandes discrètes. La plus fine et la plus récente de ces approches est la «k distribution method and the correlated k approximation» de Kato et al. (1999). Elle fournit des estimations de l'éclairement dans 32 bandes spectrales. Pour atteindre l'objectif de la thèse, nous avons tout d'abord étudié la précision de cette approche dans chacune des 32 bandes. Les résultats sont bons dans toutes les bandes excepté l'UV. Nous avons proposé une nouvelle paramétrisation de la transmittance de l'ozone, que nous avons intégré au code numérique de transfert radiatif libRadtran. Enfin, nous avons élaboré deux techniques de ré-échantillonnage permettant d'obtenir à partir des 32 valeurs discrètes, des estimations de l'éclairement dans n'importe quel intervalle spectral, y compris en prenant en compte une réponse spectrale du système éclairé. Des validations ont pu être faites à l'aide de quelques stations de mesures. Les premiers résultats sont jugés encourageants en comparaison avec la précision relevée sur les instruments de mesure de qualité. Des voies d'améliorations ont pu être recensées pour un gain de précision notable.We are interested in the estimation of the spectral distribution of the solar irradiance on the ground in any place and time. Given the very small number of measurement stations on the ground, we suggest developing a digital method exploiting estimations of the atmospheric composition made daily. This method must be enough fast to be used in intensive mode in an operational way. The targeted fields of application are the ultraviolet (UV), photosynthesis and daylight; with a limitation in the case of the clear sky. Our first innovation is to leave paradoxically from very rapid numerical approaches of calculation of the total irradiance. These developed approaches give a description of the spectral distribution of solar irradiance in a number restricted by discrete bands. The finest and the most recent of these approaches is «k-distribution method and the correlated-k approximation» of Kato and al. (1999). It supplies estimations of the irradiance in 32 spectral bands. To achieve this PhD goal, we have first of all, studied the accuracy of the approach in each of 32 bands. The results are good in all the bands except in the UV. We proposed a novel parametrization of the transmittance of the ozone, which we integrated into the radiative transfer model libRadtran. Finally, we developed two techniques of re-sampling allowing to obtain from 32 discrete values, estimations of the irradiance in any spectral interval, including by taking into account a spectral response of the illuminated system. Experimental validations have been carried out by means of some measurement stations. The preliminary results are considered encouraging in comparison with the precision found on quality measuring instruments. Ways of improvements have been listed for a gain of notable precision

Vers une méthode automatique d'estimation de la distribution spectrale du rayonnement solaire. Cas du ciel clair. : Applications à la lumière du jour, photosynthèse et ultraviolet

We are interested in the estimation of the spectral distribution of the solar irradiance on the ground in any place and time. Given the very small number of measurement stations on the ground, we suggest developing a digital method exploiting estimations of the atmospheric composition made daily. This method must be enough fast to be used in intensive mode in an operational way. The targeted fields of application are the ultraviolet (UV), photosynthesis and daylight; with a limitation in the case of the clear sky. Our first innovation is to leave paradoxically from very rapid numerical approaches of calculation of the total irradiance. These developed approaches give a description of the spectral distribution of solar irradiance in a number restricted by discrete bands. The finest and the most recent of these approaches is «k-distribution method and the correlated-k approximation» of Kato and al. (1999). It supplies estimations of the irradiance in 32 spectral bands. To achieve this PhD goal, we have first of all, studied the accuracy of the approach in each of 32 bands. The results are good in all the bands except in the UV. We proposed a novel parametrization of the transmittance of the ozone, which we integrated into the radiative transfer model libRadtran. Finally, we developed two techniques of re-sampling allowing to obtain from 32 discrete values, estimations of the irradiance in any spectral interval, including by taking into account a spectral response of the illuminated system. Experimental validations have been carried out by means of some measurement stations. The preliminary results are considered encouraging in comparison with the precision found on quality measuring instruments. Ways of improvements have been listed for a gain of notable precision.Nous nous intéressons à l'estimation de la distribution spectrale de l'éclairement solaire au sol en tout lieu et tout instant. Etant donné le très petit nombre de stations de mesure au sol, nous nous proposons de développer une méthode numérique exploitant des estimations de la composition atmosphérique faites quotidiennement. Cette méthode doit être suffisamment rapide pour être utilisée en mode intensif de manière opérationnelle. Les domaines d'applications ciblés sont l'ultraviolet (UV), la photosynthèse et la lumière du jour, avec une limitation au cas du ciel clair. Notre première innovation est de partir paradoxalement des approches numériques très rapides et précises de calcul de l'éclairement total. Ces approches développées donnent une description de la distribution spectrale du rayonnement solaire en un nombre restreint de bandes discrètes. La plus fine et la plus récente de ces approches est la «k distribution method and the correlated k approximation» de Kato et al. (1999). Elle fournit des estimations de l'éclairement dans 32 bandes spectrales. Pour atteindre l'objectif de la thèse, nous avons tout d'abord étudié la précision de cette approche dans chacune des 32 bandes. Les résultats sont bons dans toutes les bandes excepté l'UV. Nous avons proposé une nouvelle paramétrisation de la transmittance de l'ozone, que nous avons intégré au code numérique de transfert radiatif libRadtran. Enfin, nous avons élaboré deux techniques de ré-échantillonnage permettant d'obtenir à partir des 32 valeurs discrètes, des estimations de l'éclairement dans n'importe quel intervalle spectral, y compris en prenant en compte une réponse spectrale du système éclairé. Des validations ont pu être faites à l'aide de quelques stations de mesures. Les premiers résultats sont jugés encourageants en comparaison avec la précision relevée sur les instruments de mesure de qualité. Des voies d'améliorations ont pu être recensées pour un gain de précision notable

A new approach for estimating Photosynthetically Active Radiation in clear sky conditions

International audiencePhotosynthetically Active Radiation (PAR) is a determinant part of surface solar irradiance used by the plants in the process of photosynthesis. This latter promotes the growth of the plants contributing significantly to the production of biomass that can be used itself for energy production. MINES ParisTech is being developing a new approach to estimate the PAR irradiance in clear sky conditions. Its concept is based on the k-distribution method and correlated-k approximation of Kato et al. (1999) which allows fast and still accurate computations. It provides the spectral distribution of solar irradiance in 32 discrete wavelength intervals between 240 nm and 4606 nm including the PAR band. The innovation in this approach is the use of only twelve spectral calculations from the 32 correlated-k results instead of about three hundred calculations when detailed spectral calculations are made. The PAR derived by this new method is compared to detailed calculations made with the radiative transfer model libRadtran on the direct and global PAR radiation. The relative bias and the relative RMSE are close to 0.5%. The RMSE for the PAR irradiance respectively for the Photosynthetic Photon Flux Density is close to 0.5 W/m2 and 2.5 µmol/m2 s. The correlation coefficient is 0.99. It is also shown that this method provides better results than two state-of-the-art empirical methods. This work demonstrates that this new method is able to assess the PAR radiation in clear sky conditions with high accuracy

Further validation of the estimates of the downwelling solar radiation at ground level in cloud-free conditions provided by the McClear service: the case of Sub-Saharan Africa and the Maldives Archipelago

International audienceAbstract. Being part of the Copernicus Atmosphere Monitoring Service (CAMS), the McClear service provides estimates of the downwelling shortwave irradiance and its direct and diffuse components received at ground level in cloud-free conditions, with inputs on ozone, water vapor and aerosol properties from CAMS. McClear estimates have been validated over several parts of the world by various authors. This article makes a step forward by comparing McClear estimates to measurements performed at 44 ground-based stations located in Sub-Saharan Africa and the Maldives Archipelago in the Indian Ocean. The global irradiance received on a horizontal surface (G) and its direct component received at normal incidence (BN) provided by the McClear-v3 service were compared to 1 min measurements made in cloud-free conditions at the stations. The correlation coefficient is greater than 0.96 for G, whereas it is greater than 0.70 at all stations but five for BN. The mean of G is accurately estimated at stations located in arid climates (BSh, BWh, BSk, BWk) and temperate climates without a dry season and a hot or warm summer (Cfa, Cfb) or with a dry and hot summer (Csa) with a relative bias in the range [−1.5, 1.5] % with respect to the means of the measurements at each station. It is underestimated in tropical climates of monsoon type (Am) and overestimated in tropical climates of savannah type (Aw) and temperate climates with a dry winter and hot (Cwa) or warm (Cwb) summer. The McClear service tends to overestimate the mean of BN. The standard deviation of errors for G ranges between 13 W m−2 (1.3 %) and 31 W m−2 (3.7 %) and that for BN ranges between 31 W m−2 (3.0 %), and 70 W m−2 (7.9 %). Both offer small variations in time and space. A review of previous works reveals no significant difference between their results and ours. This work establishes a general overview of the performances of the McClear service

A New Clear-Sky Method for Assessing Photosynthetically Active Radiation at the Surface Level

A clear–sky method to estimate the photosynthetically active radiation (PAR) at the surface level in cloudless atmospheres is presented and validated. It uses a fast and accurate approximation adopted in several radiative transfer models, known as the k-distribution method and the correlated-k approximation, which gives a set of fluxes accumulated over 32 established wavelength intervals. A resampling technique, followed by a summation, are applied over the wavelength range [0.4, 0.7] µm in order to retrieve the PAR fluxes. The method uses as inputs the total column contents of ozone and water vapor, and optical properties of aerosols provided by the Copernicus Atmosphere Monitoring Service. To validate the method, its outcomes were compared to instantaneous global photosynthetic photon flux density (PPFD) measurements acquired at seven experimental sites of the Surface Radiation Budget Network (SURFRAD) located in various climates in the USA. The bias lies in the interval [−12, 61] µmol m−2 s−1 ([−1, 5] % in values relative to the means of the measurements at each station). The root mean square error ranges between 37 µmol m−2 s−1 (3%) and 82 µmol m−2 s−1 (6%). The squared correlation coefficient fluctuates from 0.97 to 0.99. This comparison demonstrates the high level of accuracy of the presented method, which offers an accurate estimate of PAR fluxes in cloudless atmospheres at high spatial and temporal resolutions useful for several bio geophysical models

A hybrid method for reconstructing the historical evolution of aerosol optical depth from sunshine duration measurements

A novel method has been developed to estimate aerosol optical depth (AOD) from sunshine duration (SD) measurements under cloud-free conditions. It is a physically based method serving for the reconstruction of the historical evolution of AOD during the last century. In addition to sunshine duration data, it requires daily water vapor and ozone products as inputs taken from the ECMWF 20th century reanalysis ERA-20C, available at the global scale over the period 1900–2010. Surface synoptic cloud observations are used to identify cloud-free days. For 16 sites over Europe, the accuracy of the estimated daily AOD, and its seasonal variability, is similar to or better than those from two earlier methods when compared to AErosol RObotic NETwork measurements. In addition, it also improves the detection of the signal from massive aerosol events such as important volcanic eruptions (e.g., Arenal and Fernandina Island in 1968, El Chichón in 1982 and Pinatubo in 1992). Finally, the reconstructed AOD time series are in good agreement with the dimming/brightening phenomenon and also provide preliminary evidence of the early-brightening phenomenon.ISSN:1867-1381ISSN:1867-854